The invention concerns a method and a device for measuring cell voltages of accumulator cells in a plurality of series-connected accumulator cells as they are used, for example, in so-called battery packs or cell packs such as Li-ion cell packs. The invention concerns moreover a charging device for a battery pack comprising such a device.
Typical uses of these rechargeable battery packs are, for example, electric power tools, hand-held household devices, for example, vacuum cleaners, flashlights, cleaning devices, or other devices for gardening, tree and brush pruning.
Battery packs of this and other kind contain in general a plurality of series-connected individual accumulator cells. In particular in case of Li-ion battery packs there is often the need to measure the voltages across the individual series-connected (i.e. serially-connected) cells (so-called individual or cell voltage) in order to be able, for example, to determine capacitance states of the battery pack or to optimally control the charge/discharge cycles.
Measuring these cell voltages is done generally relative to a common potential, in particular, the negative terminal of the serial connection or of the battery pack. In this connection, often the problem is encountered that during the measurements of the cell voltages the individual cells are loaded at different levels and therefore become unbalanced. It has been found that, as a result of this unbalance, the effective capacitance of the battery pack may be reduced.
For example, in DE 103 47 110 a device and a method for measuring individual cell voltages in a cell stack of an energy storage device is disclosed. In this connection, parallel to each cell a serial connection of two diodes is arranged whose connecting points are connected, respectively, by a capacitor and a switch to a differential amplifier. An alternating current of a certain frequency and amplitude is applied to the capacitors and generates alternating voltage in accordance with the cell voltage that, after rectification, is converted to a ground-based direct current value for the cell voltage.
DE 103 20 127 discloses a control device and a control method for a storage battery in which, respectively, are provided a first control unit for releasing charges stored in a storage battery (capacitor) in ca se that the terminal voltage at the storage battery is at or above a predetermined threshold value; a temperature detection device for detecting the temperature of the storage battery; as well as a second control device that changes the predetermined threshold value as a function of the temperature of the storage battery. In that, in case of a change of the terminal voltage of the storage battery caused by a temperature change, the threshold value is also changed, an unnecessary discharge is to be prevented and the stored charge quantity is to be increased.
DE 100 51 984 discloses a battery voltage measuring device in particular for such battery cells that are combined of a plurality of series-connected modules each comprising a plurality of series-connected secondary batteries. The device comprises substantially for each module a capacitor, that is charged to the voltage of the associated module; switches that connect and disconnect this capacitor and the module; a voltage follower that outputs the voltage across the capacitor, as well as switches that connect and disconnect the voltage follower and the capacitor. In this way, the battery voltage is to be measured at high precision.
EP 1 750 139 discloses a voltage measuring device for a battery cell group that is combined of a plurality of series-connected battery cells. The measuring device comprises essentially a sampling switch for connecting one or several battery cells to be measured with a capacitor so that the latter is charged by these battery cells; a measuring component that detects and corrects the voltage at the charged capacitor as a measuring voltage so that the voltage error that is caused in the measured voltage by the connecting capacitance of the sampling switch is reduced; as well as an overvoltage protective switch that is activated immediately before switching on the sampling switch and that grounds the input terminal of the measuring component so that the measuring component is protected from overvoltage. In this way, essentially the measuring precision is to be increased and it is to be prevented that overvoltage can reach the measuring component.
Finally, US 2004/0051534 discloses a further voltage measuring device for a battery cell group that is comprised of a plurality of series-connected battery cells. A number of series-connected capacitors that are connectable by switches alternatively to different cells serve for storing the cell voltages that are supplied through analog protective switches to a voltage detector in A/D converted form. Moreover, noise suppression is provided also. In this way, the battery voltage is to be measured with minimal measuring error in a reliable way.
One object upon which the invention is based resides therefore in that a method and a device for measuring cell voltages of accumulator cells in a plurality of series-connected accumulator cells (in particular in a battery pack) are to be provided with which the unbalance of the serial cell connection that is caused by the aforementioned measurements of the cell voltages at the individual cells can be at least substantially reduced.